The present invention relates to an abnormal noise detector for use in the inspection of gear units. More particularly, it relates to an abnormal noise detector which can detect flaws in the surface of the teeth of a gear in a gear unit by detecting the abnormal noises resulting from those flaws.
One of the typical sources of abnormal noise from a gear unit is a gear tooth having a marred surface. When such a gear tooth meshes with the teeth of a second gear, an abnormal noise is produced. If the rate of rotation of the gear with the marred tooth is N revolutions per second, the marred tooth will mesh N times per second and an abnormal noise will be produced at intervals of 1/N seconds.
The abnormal noise can manifest itself in several ways. One is in the form of a momentary variation in the sound pressure level of the noise produced by the meshing of the gear with a marred tooth and a second gear. Of all the different noises produced during the operation of a gear unit, the noise having the greatest sound pressure level is that due to the meshing of the gears. For any two gears in a gear unit, the fundamental frequency of this noise is equal to the rate at which the teeth of the two gears mesh. This fundamental frequency will here be referred to as f.sub.0 and is equal to (N)(N.sub.G), where N.sub.G is the number of gears teeth in the gear with the marred tooth surface. As shown in FIG. 1a, which is a graph of the variation over time of the sound pressure level of the noise having the fundamental frequency f.sub.o, the abnormal noise produced by a marred gear tooth can manifest itself as peaks in the sound pressure level occurring at intervals of 1/N seconds. The width of each peak is equal to the length of time for which the tooth with the marred surface meshes with the teeth of a second gear, and is thus equal to 1/(N.times.N.sub.G) seconds=1/f.sub.0. A similar variation in sound pressure level can be observed for noises due to meshing whose frequencies are integral multiples of the fundamental frequency f.sub.0.
Another way in which the abnormal noise resulting from a marred gear tooth can manifest itself is in the form of a variation in the fundamental frequency of the noise due to meshing of the gears in the gear unit. In a normal gear, this frequency is always equal to f.sub.0 =(N)(N.sub.G). However, when a marred tooth meshes with a second gear, a slight change in the timing of the gear unit can be produced, and this results in a momentary slight variation alpha in the fundamental frequency, as shown in FIG. 1b. This variation occurs at intervals of 1/N seconds (once per revolution), and the width of each variation in frequency is 1/(N.times.N.sub.G) seconds, the same as the width of the peaks in sound pressure level illustrated in FIG. 1a.
The conventional method for detecting abnormal noises produced by a gear unit is to employ a microphone and a frequency analyzer. A gear unit to be tested is connected between a drive motor and a load such as a second motor, and the gear unit is driven by the drive motor at various speeds. The noise produced by the gear unit is picked up by the microphone which produces a corresponding electrical output signal, and the output signal of the microphone is then provided to the frequency analyzer as an input signal. The frequency analyzer samples the signal from the microphone for a certain period of time T and produces a sound pressure level spectrum representing the average sound pressure level of each frequency of noise produced by the gear unit during the sampling period T at the given rate of rotation. By comparing the sound pressure level spectrum for a gear unit being tested with a standard sound spectrum for a gear unit operating normally, it is possible to detect certain abnormalities in the sound pressure level spectrum due to flaws in the gear unit being tested.
However, this method of detecting abnormal noises is not appropriate for detecting the abnormal noise phenomena illustrated in FIG. 1a and 1b resulting from a gear tooth with a marred surface. One or both of these phenomena may be produced by a given gear unit, and they may be produced continuously or intermittently, depending on the gear unit. Even if both phenomena are produced by a gear unit, they are not necessarily produced simultaneously. Due to the intermittent nature of these abnormal noise phenomena and their very short pulse widths, i.e. 1/(N.times.N.sub.G) seconds, they are very difficult to detect and analyze using the conventional method of abnormal noise detection using only a microphone and a frequency analyzer. Namely, if the sampling period T of the frequency analyzer is very short, it is easy for the abnormal noise phenomena to occur outside of the sampling period, due to their intermittent nature. On the other hand, if the sampling period T is made long, the variations in sound pressure level and frequency tend to be averaged out, and it is difficult to ascertain noise abnormalities from the resulting data.